An apparatus for determining the degree of water saturation of the hygroscopic desiccant of a disposable drying cartridge as a function of the variation in the dielectric constant of the hygroscopic material. The invention takes the form of a tubular capacitor disposed in the mass of hygroscopic desiccant material proximate the outlet of the drying cartridge, and of appropriate circuits for measuring the level of saturation of the desiccant material and for operating alarm and safety devices. A removable plug provides access to the cartridge mounted in a casing. The plug has a pair of electrical contacts acting to biasing position the cartridge in the casing and provide effective electrical contact with the capacitor.